The present invention relates generally to stents which are implantable or deployable in a vessel or duct within the body of a patient to maintain the lumen of the duct or vessel open, and more particularly to an improved highly flexible, low friction stent structure.
Stents are implantable by deployment in a vessel or duct within the body of a patient to maintain the patency (unblocked or unclogged characteristic) of the lumen of the duct or vessel; i.e., to keep the vessel open. The stent itself is a tubular, perforated wall, open-ended, expandable vascular or endoluminal prosthesis. Although it has enjoyed widespread use principally to keep a designated target site of the lumen in a blood vessel open and unoccluded, especially in the coronary and femoral arteries following angioplasty, the device has found increasing use for the same or similar purpose in other places in the human body. Examples include maintaining the lumen open and unobstructed at a preselected target site in the tracheobronchial system, the biliary hepatic system, the esophageal bowel system, or the urinary tract system.
A vascular stent, in particular, must be of sufficient dimensional stability to keep the lumen of the vessel open while resisting recoil of its elastic wall that naturally occurs when the target site within the vessel or luminal structure has been subjected to outwardly directed forces necessary to expand the elastic fibers during deployment of the stent. It was found that a large percentage of patients who had undergone an angioplasty procedure were returning with blockage of the same coronary artery within three to six months after the angioplasty was performed. It was subsequently discovered that the new blockage was attributable to a different mechanismxe2x80x94the trauma to the artery wall during the angioplasty procedure had caused a proliferation of smooth muscle cells (hyperplasia) which constituted restenosisxe2x80x94in this case somewhat akin to scarring. Implantation of a coronary stent can serve not only to reduce acute complications following an angioplasty intervention, but also improve the long term outcome, such as to inhibit restenosis and suppress or limit recoil by the stent""s scaffolding and support of the vessel wall.
Among the different stent designs are a wire mesh type, a coil type with a helical wire configuration, a slotted tube type, and a multicellular type which is a modification of the slotted tube type with less surface coverage and smaller openings. Typically, the stent is implanted from a delivery system which includes a catheter, a balloon generally at or near the distal end of the catheter, and an inflation lumen in the catheter for selectively inflating and deflating the balloon with a suitable biologically compatible fluid, the stent being crimped onto the balloon. The balloon catheter with stent must, of course, have a diameter smaller than the diameter of the vessel in which the stent is to be implanted. A coronary artery may have a diameter of only about 3 millimeters. The catheter is inserted from its proximal end into the vessel and advanced until the stent is aligned (as viewed under fluoroscope by the implanting physician) at the target site, such as a section of a coronary artery which has just been treated with an angioplasty procedure, and the stent is then deployed by inflating the balloon to expand the stent diameter, whereby the stent engages and at least slightly expands the lumen diameter of the vessel wall.
In addition to adequate support strength in the deployed statexe2x80x94sometimes referred to as mechanical scaffolding, hoop strength or radial strengthxe2x80x94to resist vessel wall recoil and to maintain the vessel patency, the stent also must be sufficiently flexible to be advanced through the lumen of an often narrow and tortuous vessel on its delivery system catheter without injuring or distending the vessel wall. Indeed, the stent must have a capacity to resist and yet flex with the repetitive pressures exerted on it by the coronary artery wall according to the systole and diastole of a beating heart. It is therefore necessary that some compromise be reached between these two conflicting requirements. The ""600 patent discloses a composite stent design pattern of interconnected struts and openings therebetween in the stent""s tubular wall which is different along its mid-section from either of its end segments, giving the stent greater rigidity at its mid-section and greater flexibility at its ends. The more rigid mid-section can better withstand recoil and repetitive pressure of the vessel wall when the stent is deployed. The more flexible ends allow the undeployed (crimped, or compressed) stent to better traverse tortuous paths encountered during advancement through the lumen of the vessel, and the deployed stent to accommodate repetitive wall flexation. Also, the more flexible ends provide a smooth transition between the native vessel wall and the more rigid mid-section, so as to match the biomechanics of the vessel itself
A coronary (vascular) stent must be implanted rapidly, to avoid the possibility of subjecting the patient to risk of myocardial infarction owing to the reduction or even complete blockage of blood flow through the coronary artery while the stent delivery system is being navigated through the vascular system and ultimately deployed at the target site. This imposes even greater importance on axial or longitudinal flexibility of the stent, as well as the skill of the implanting physician. Additionally, it is crucial that the stent exhibit low surface friction. Many of the body vessels, tracts or ducts through which a stent must be advanced to reach the target site exhibit a surface which is not smooth, but rather, uneven, calcified or stenosed.
Therefore, an ideal stent must be structured to minimize the impact of surface friction along the path it must traverse, as well as possess features of longitudinal flexibility and mechanical scaffolding. Low surface friction is especially important in the compressed state or condition of the stent when it is mounted on the balloon catheter of the delivery system, for it is in this condition that the stent must be guided through the vessel. It is unacceptable for the stent structure itself to exacerbate the problem of friction along the path, by presenting a compressed state whose surface friction characteristics, coupled with longitudinal bending of the stent that must take place during advancement through a curved vessel, creates hook-like anomalies at the outer surface of the stent.
It is a principal aim of the present invention to provide a stent having structural characteristics of high longitudinal flexibility, strong mechanical scaffolding, and low surface friction, compared to previous stent designs.
In heretofore available stent designs, whether of the mesh, coil, slotted or multicellular type, it has been customary to provide transversely or laterally oriented structural elements (relative to the longitudinal axis of the stent) that interconnect longitudinally oriented elements in the stent structure. An extreme example is the coil stent, in which a single element (the coil itself) provides both longitudinal and transverse orientation relative to the direction of advancement of the stent through the vessel or duct. Transverse elements or portions of a stent structure tend to exacerbate surface friction during advancement (or withdrawal) of the stent through the vessel, particularly if the stent undergoes longitudinal bending as it traverses a tortuous vessel. In general, bending becomes more pronounced as stent length increases.
Therefore, another aim of the present invention is to provide a stent structure of the slotted tube or multicellular type in which the structural elements are oriented or aligned in a predominantly (i.e., virtually entirely) longitudinal direction relative to the axis of the stent.
An additional factor which makes stent structural elements of predominantly longitudinal orientation a functionally desirable design is that the stent is more readily compressed onto the uninflated balloon of the delivery system with a small profile. Presently available catheter-mounted balloons offer a minimum uninflated diameter in the range from about 0.6 to 0.7 millimeters, which dictates a minimum circumference of about 2.0 mm. A stent compressed onto the balloon should not measurably increase that minimum circumference, so the compressed stent preferably should not exceed a circumference of about 2.0 mm. This tends to assure passage of the stent through a small diameter coronary artery, prevention against the stent being dislodged from the balloon as the catheter is advanced to the target site in the artery. Transversely oriented structural elements much more profoundly limit the extent to which the stent may be compressed as it is crimped onto a low profile balloon, than do longitudinally oriented structural elements.
Accordingly, another objective of the invention is to provide a stent with optimum longitudinal orientation of all its structural elements, to permit the stent to be compressed to a diameter of less than 1.0 mm onto a delivery balloon, and to reduce the surface friction characteristics of the stent for more rapid advancement through a narrow diameter vessel.
Briefly, according to the invention the stent has the customary generally cylindrical, open-ended, tubular structure with a longitudinal axis, but in its production (i.e., completed manufacture) state, as well as its radially compressed state or even its partly expanded (i.e., pre-opened) state, has a self-supporting latticework sidewall with predominantly longitudinally oriented elements (struts, links or strips). Each of these interconnected struts has the predominantly longitudinal orientation relative to the axis of the stent, with none having a predominantly transverse orientation relative to that axis. The effect of this design of the latticework sidewall is to optimize its outer surface for low friction when the stent is being advanced or withdrawn through the duct or vessel to or from a target site.
The sidewall is of generally uniform thickness, with a multiplicity of holes therethrough that enable the stent to be selectively expanded radially during inflation of the delivery balloon when the stent is being deployed. In its expanded state, in which the stent is adapted to engage the wall of the vessel at the target site, a considerable number of the struts in the sidewall are deformed, during stent deployment and by virtue of the structural design and composition of the stent, from the longitudinal orientation to a transverse orientation that provides a self-supporting, mechanical scaffolding sufficient to resist radial compression under forces exerted by recoil of the vessel wall and ongoing repetitive flexing in the case of a blood vessel.
In a presently preferred embodiment, a vascular or endoluminal stent comprises a biocompatible hollow tube having a longitudinal axis and open ends, a multiplicity of openings through the wall of the tube between the ends, the stent having a production state, a second state in which the stent is radially compressed and a third state in which the stent is radially expanded relative to the production state. The stent is adapted for deployment to its expanded state in a vessel, duct or tract of a patient. The multiplicity of openings through the wall of the tube is defined when the stent is in the production state by a network of tangentially interconnected, solely curvilinear struts, each of the struts running longitudinally from end to end of the tube in repetitively alternating crests and valleys without sharp breaks or angularity.
Each of the openings is bounded circumferentially on the tube by an upper curve and a lower curve connected to form a closed curve. One of these upper and lower curves has a tighter curvature than the other. As viewed in one aspect, each opening in the wall of the stent has a shape resembling the outline of a ram""s head with horns projecting outwardly and upwardly at sides of the head. In this aspect, each of the upper and lower curves of each opening has a single valley or trough. As viewed in another aspect, each opening in the wall of the stent has a shape resembling the outline of a handlebar moustache or a Dutch winged cap. In that aspect, each of the upper and lower curves of each opening has a single crest.
In either case, no segment of any strut is oriented in a direction generally perpendicular to the longitudinal axis when the stent is in either its production state or its compressed state. But when deployed to the expanded state, each strut has at least one segment oriented in a direction generally perpendicular to the longitudinal axis. The tube that forms the stent is longitudinally flexible to undergo a bend defining an inner arc and an outer arc, wherein the openings closest to the inner arc have upper and lower curves closer together than the upper and lower curves of circumferentially aligned openings closest to the outer arc.
The stent is composed of a material such that the circumferentially aligned openings in the tube wall from the inner arc to the outer arc will undergo recovery toward their respective original configurations upon straightening of the bend, when the bend has occurred with the stent in its compressed state as would be the case during navigation of the stent (mounted on its delivery system) through a curved vessel. However, when the bend is an accommodation to a curve at the target site during deployment of the stent to its expanded state, the material undergoes plastic deformation such that the circumferentially aligned openings in the tube wall from the inner arc to the outer arc will remain in respective configurations of upper and lower curves being closer together or further apart according to their closeness to the inner arc or outer arc, respectively.